Touch screens are the growing trend for input devices to a wide range of computing devices, especially smart phones. A certain class of touch sense arrays includes a two-dimensional array of capacitors, referred to as sense elements. Touch sense arrays can be scanned in several ways, one of which (mutual-capacitance sensing) permits individual capacitive elements to be measured. Another method (self-capacitance sensing) can measure an entire sensor strip, or even an entire sensor array, with less information about a specific location, but performed with a single read operation.
The two-dimensional array of capacitors, when placed in close proximity, provides a means for sensing touch. A conductive object, such as a finger or a stylus, coming in close proximity to the touch sense array causes changes in the capacitances of the sense elements in proximity to the conductive object. More particularly, when a finger touch occurs, self capacitance increases whereas mutual capacitance decreases. These changes in capacitance can be measured to produce a “two-dimensional map” that indicates where the touch on the array has occurred.
One way to measure such capacitance changes is to form a circuit comprising a signal driver (e.g., an AC current or a voltage source) which is applied to each horizontally aligned conductor in a multiplexed fashion. The charge associated with each of the capacitive intersections is sensed and similarly scanned at each of the vertically aligned electrodes in synchronization with the applied current/voltage source. This charge is then measured using a slot-by-slot touch sense controller that typically includes a form of charge-to-voltage converter, followed by a multiplexor of sense electrodes and A/D converter that is interfaced with a CPU to convert the input signal to digital form for input to a processor. The processor, in turn, renders the “two-dimensional map” or “touch map” and determines the location of a touch. A full slot-by-slot scan of each of the slots is performed before the resulting data is processed by the CPU in a serial fashion.
Current smart phones and tablet computers require such features as gesture, fat finger, and finger identifier (ID) sensing, as well as operating system (OS) support, fast performance, high signal-to-noise ratio (SNR), and high resolution at a low price. Conventional serial scanning and processing techniques implemented in touch sense controllers are inadequate.